Detecting thickness variation and quantitative depth utilizing scanning electron microscopy with a surface profiler

ABSTRACT

A method and system to detect thickness variation of a subject material are described. In an aspect, tribological wear is assessed for a disk drive memory system at the pole tip region of a magnetic head. Images are obtained of a first region and a second region of a subject material utilizing scanning electron microscopy (SEM). The SEM images are image processed to obtain a differential contrast between the first region and the second region. An image intensity variation is determined between masked SEM images of the first region and the second region by obtaining a surface profiler image of the first region and the second region, and overlaying and calibrating the SEM images with the surface profiler images. In an aspect, image intensity variation is converted to quantified thickness utilizing a fitted relation obtained from the calibration of the surface profiler images with the SEM images.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/310,294, filed Jun. 20, 2014, the entire content of which isincorporated herein by reference.

BACKGROUND

A scanning electron microscope (SEM) is used to generate high-resolutionimages of objects and to show spatial variations. A SEM uses a focusedbeam of high-energy electrons to generate a variety of signals at thesurface of solid specimens. In applications, data are collected over aselected area of the surface of the sample, and a two-dimensional imageis generated that displays spatial variations.

While a SEM is useful for a variety of applications, one application inwhich it is used is with magnetic storage systems to qualitatively andsubjectively assess head-media interactions. Magnetic storage systemsare utilized in a wide variety of devices in both stationary and mobilecomputing environments. Magnetic storage systems include hard diskdrives (HDD), and solid state hybrid drives (SSHD) that combine featuresof a solid-state drive (SSD) and a hard disk drive (HDD). Examples ofdevices that incorporate magnetic storage systems include desktopcomputers, portable notebook computers, portable hard disk drives,servers, network attached storage, digital versatile disc (DVD) players,high definition television receivers, vehicle control systems, cellularor mobile telephones, television set top boxes, digital cameras, digitalvideo cameras, video game consoles, and portable media players.

Hard disk drive performance demands and design needs have intensified. Ahard disk drive typically includes a read head and a write head,generally a magnetic transducer which can sense and change magneticfields stored on disks. The current demand for larger capacity in asmaller dimension is linked to the demand for ever increasing storagetrack density. The seek time is the time it takes the head assembly totravel to a disk track where data will be read or written. The time toaccess data can be improved by reducing seek time, which affects HDDperformance. Reduced seek time and very close spacing between the headsand the disk surface make HDDs vulnerable to damage caused by head-mediacontact, which may cause data loss. While head-media contact can resultin immediate head and media failure or data loss, repeated head-mediacontact can result in eventual head and media degradation, includingdiamond-like carbon (DLC) wear at the air bearing surface, depletion ofmedia surface lubrication, and scratches to media surface, which canalso result in head and media failure or data loss.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages describedherein will become more readily appreciated by reference to thefollowing detailed description, when taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a top plan view of a disk drive data storage system in whichembodiments are useful;

FIG. 2 is a flow diagram illustrating a method or process for detectingthickness variation of a subject material, in an embodiment, and furtherquantifying thickness depth in another embodiment, as can be used in adata storage system as in FIG. 1, in an embodiment;

FIG. 3 is a representative graph illustrating experimental data of SEMimages before background leveling, and after background leveling, in anembodiment;

FIG. 4 is a representative graph illustrating experimental data of imageintensities mapping for contrast normalization, in an embodiment;

FIG. 5 are representative graphs illustrating experimental output of apre-processed SEM image, a SEM mask of wear, points of detected wear,and a wear map, in an embodiment;

FIG. 6 is a sectional view representation illustrating components of asystem that executes methods of an embodiment; and

FIG. 7 is a representative graph illustrating experimental data ofquantitative wear for example HDD shields, in an embodiment.

DETAILED DESCRIPTION

In the following description, numerous specific details are disclosed toprovide a thorough understanding of embodiments of the method, systemand apparatus. One skilled in the relevant art will recognize, however,that embodiments of the method, system and apparatus described hereinmay be practiced without one or more of the specific details, or withother electronic devices, methods, components, and materials, and thatvarious changes and modifications can be made while remaining within thescope of the appended claims. In other instances, well-known electronicdevices, components, structures, materials, operations, methods, processsteps and the like may not be shown or described in detail to avoidobscuring aspects of the embodiments. Embodiments of the apparatus,method and system are described herein with reference to figures.

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, electronic device, method or characteristic described inconnection with the embodiment is included in at least one embodiment.Thus, appearances of the phrases “in one embodiment,” “in anembodiment,” and similar language throughout this specification mayrefer to separate embodiments or may all refer to the same embodiment.Furthermore, the described features, structures, methods, electronicdevices, or characteristics may be combined in any suitable manner inone or more embodiments.

The interaction of an electron beam with matter in an electronmicroscope generates a multitude of signals which can be used tocharacterize physical and chemical properties of a sample underanalysis. Of these signals, two primary signals are secondary andbackscattered electrons. While secondary electrons are more sensitive tosurface topography variation and provide contrast correspondent thereof,backscattered electrons are sensitive to material properties at theatomic scale and therefor provide material contrast.

The variation of backscattered electron yield as a function of atomicnumber may be obtained from scientific references. The backscatteredelectron contrast between two regions is described by the followingequation, where ε_(BS) is the efficiency with which backscatteredelectrons are detected and η is the backscatter coefficient of thematerials.

$C = {\frac{\eta_{2} - \eta_{1}}{\eta_{2}} = {\frac{S_{2} - S_{1}}{S_{2}} = \frac{{ɛ_{{BS}_{2}}\eta_{2}} - {ɛ_{{{BS}\;}_{1}}\eta_{1}}}{ɛ_{{BS}_{2}}\eta_{2}}}}$

A Monte Carlo simulation of the backscatter coefficient for a carbonthin film on a NiFe substrate for various carbon film thicknesses showsthat as carbon thickness decreases, the backscatter coefficientincreases. As an example, the following is observed: for a 10 nm carbonthickness the backscatter coefficient is 0.075, for a 8 nm carbonthickness the backscatter coefficient is 0.093, for a 6 nm carbonthickness the backscatter coefficient is 0.0126, for a 4 nm carbonthickness the backscatter coefficient is 0.181, for a 2 nm carbonthickness the backscatter coefficient is 0.245, for a 1 nm carbonthickness the backscatter coefficient is 0.272, and for a 0 nm carbonthickness the backscatter coefficient is 0.03. From the Monte Carlosimulation, it can be derived that the electron backscatter coefficientvaries nearly linearly over a large range of carbon thicknesses forcarbon on a NiFe substrate. The backscatter coefficient varies nearlylinearly especially in the 1-2 nm thickness regime, for example, forcarbon on a NiFe substrate as used in magnetic recording head overcoats.

Scanning electron microscopy may be used to assess surfaces of magneticstorage systems including hard disk drives (HDD), and solid state hybriddrives (SSHD). It has been used with magnetic storage systems toqualitatively and subjectively assess head-media interactions, such aswear and lubricant pickup.

Thermo-mechanical interaction of a magnetic recording head or slider,with media or a disk, fundamentally affects hard disk drive (HDD)performance and reliability. A diamond-like carbon (DLC) overcoat isused as a protective layer for head-media interaction, and also providesa corrosion barrier for the head. The magnetic head includes a shieldcomprised of a nickel iron alloy that is susceptible to corrosion. Theintegrity of the DLC is thus critical to HDD performance andreliability. Tribological testing at the component level is used toobtain an early understanding of these interactions and predictions ofperformance in drives.

A scanning electron microscope is conventionally used for qualitativeassessment of wear. Wear can be quantitatively measured by atomic forcemicroscopy (AFM) as a separate measurement, but because an electronmicroscope deposits carbon contamination on an imaged surface, an AFMmeasurement must be performed prior to electron microscope imaging, thusadding another step. Atomic force microscope wear measurements, withsufficient resolution to map the wear characteristics, are impracticalfor at least one reason in that the associated scan times become so longthat thermal and mechanical image drift limit measurement precision. AFMis also significantly sensitive to pre-existing topography, which causesambiguity when distinguishing wear. In conventionally used assessmentmethods, measurement of wear requires measurements before and afterbeing subjected to wear conditions. Some embodiments described hereinprovide quantitative wear assessment without conventionally neededcharacterization overhead or cycle-time.

An apparatus, system and method are described herein for detectingthickness variation of a subject material from a scanning electronmicroscope (SEM) image and another surface profiler image. An embodimentfurther provides a quantitative depth and area assessment, and in anexample application the quantitative depth assessment is provided at asub-nanometer scale. In an embodiment, the apparatus, system and methodsmay be utilized to detect thickness variation of a subject material, andquantitative depth and area assessment for numerous applications infields that would benefit from an objective thickness and areaassessment of a subject material. Applications include, among otherthings, cutting and machining tools. One application, of manyapplications, in which quantitative depth and area assessment of asubject material is useful in determining tribological wear in the caseof a disk drive memory system, or other memory systems utilizing amagnetic reading device, including a HDD and a SSHD. An embodimentprovides a two-dimensional quantitative depth assessment of tribologicalwear of a magnetic head or media. An embodiment provides quantitativedepth and area assessment of tribological wear of a magnetic head at thepole tip region of the magnetic head.

Referring to the figures wherein identical reference numerals denote thesame elements throughout the various views, FIG. 1 illustrates a diskdrive storage system 10, in which embodiments are useful. Features ofthe discussion and claims are not limited to this particular design,which is shown only for purposes of the example. Disk drive 10 includesbase plate 12 that may be disposed on a top cover forming a sealedenvironment to protect internal components from contamination.

Disk drive 10 further includes one or more data storage disks 14 ofmagnetic computer-readable data storage media. The disks are generallyformed of two main substances, namely, a substrate material that givesit structure and rigidity, and a magnetic media coating that holds themagnetic impulses or moments that represent data. Typically, both of themajor surfaces of each data storage disk 14 include a plurality ofconcentrically disposed tracks for data storage purposes. Each datastorage disk 14 is mounted on a hub or spindle 16, which in turn isrotatably interconnected with a base plate 12 and/or cover. Multipledata storage disks 14 are typically mounted in vertically spaced andparallel relation on the spindle 16. A spindle motor 18 rotates the datastorage disks 14 at an appropriate rate. Perpendicular magneticrecording (PMR) involves recorded bits that are stored in a generallyplanar recording layer in a generally perpendicular or out-of-planeorientation. A PMR read head and a PMR write head are usually formed asan integrated read/write head on an air-bearing slider.

The disk drive 10 also includes an actuator arm assembly 24 that pivotsabout a pivot bearing 22, which in turn is rotatably supported by thebase plate 12 and/or cover. The actuator arm assembly 24 includes one ormore individual rigid actuator arms 26 that extend out from near thepivot bearing 22. Multiple actuator arms 26 are typically disposed invertically spaced relation, with one actuator arm 26 being provided foreach major data storage surface of each data storage disk 14 of the diskdrive 10. Other types of actuator arm assembly configurations may beutilized as well, such as an assembly having one or more rigid actuatorarm tips or the like that cantilever from a common structure. Movementof the actuator arm assembly 24 is provided by an actuator arm driveassembly, such as a voice coil motor 20 or the like. The voice coilmotor (VCM) 20 is a magnetic assembly that controls the operation of theactuator arm assembly 24 under the direction of control electronics 40.

A suspension 28 is attached to the free end of each actuator arm 26 andcantilevers therefrom. The slider 30 is disposed at or near the free endof each suspension 28. What is commonly referred to as the read/writehead (e.g., transducer) is mounted as a head unit 32 under the slider 30and is used in disk drive read/write operations. As the suspension 28moves, the slider 30 moves along arc path 34 and across thecorresponding data storage disk 14 to position the head unit 32 at aselected position on the data storage disk 14 for the disk driveread/write operations. When the disk drive 10 is not in operation, theactuator arm assembly 24 may be pivoted to a parked position utilizingramp assembly 42. The head unit 32 is connected to a preamplifier 36 viahead wires routed along the actuator arm 26, which is interconnectedwith the control electronics 40 of the disk drive 10 by a flex cable 38that is typically mounted on the actuator arm assembly 24. Signals areexchanged between the head unit 32 and its corresponding data storagedisk 14 for disk drive read/write operations.

The data storage disks 14 comprise a plurality of embedded servo sectorseach comprising coarse head position information, such as a trackaddress, and fine head position information, such as servo bursts. Asthe head 32 passes over each servo sector, a read/write channelprocesses the read signal emanating from the head to demodulate theposition information. The control circuitry processes the positioninformation to generate a control signal applied to the VCM 20. The VCM20 rotates the actuator arm 26 in order to position the head over atarget track during the seek operation, and maintains the head over thetarget track during a tracking operation.

The head unit 32 may utilize various types of read sensor technologiessuch as anisotropic magnetoresistive (AMR), giant magnetoresistive(GMR), tunneling magnetoresistive (TMR), other magnetoresistivetechnologies, or other suitable technologies.

Magnetic recording is a near-field process in which reading and writingby the read/write head occur in close proximity to the disk surface.While head-media contact can result in immediate head and media failureor data loss, repeated head-media contact can result in eventual headand media degradation, including diamond-like carbon (DLC) wear at theair bearing surface, depletion of media surface lubrication, andscratches to media surface, which can also result in head and mediafailure or data loss.

FIG. 2 illustrates a method and process for detecting thicknessvariation of a subject material, in an embodiment, and furtherquantifying thickness depth in another embodiment. Numerous materialsmay be used as the subject material. In one application, the subjectmaterial is a surface from a data storage device, including a portion ofa magnetic read head overcoat and/or write head overcoat, or a surfaceof a data storage disk facing an air bearing surface. In an embodiment,the differential contrast is due to differential tribological wearbetween a first region and a second region of the magnetic read headovercoat and/or write head overcoat. The overcoat may be diamond-likecarbon.

In an embodiment, each step in the flowchart illustration can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a programmable dataprocessing apparatus, such that the instructions execute via theprocessor to implement the functions or actions specified in theflowchart.

As detailed in step 202, images are obtained of a first region of asubject material and a second region of a subject material, utilizingscanning electron microscopy (SEM). In an embodiment, other electronmicroscope types may be utilized including but not limited totransmission electron microscope, reflection electron microscope, andscanning transmission electron microscope. The first region is forexample a region having wear (e.g., an air bearing surface or open-fieldalumina), and the second region is for example a region not having wear(e.g., a HDD shield). At least some areas of the first region are lessthick than the second region, and the subject material is overlying asecond material. In an example embodiment, the first region of thesubject material is a HDD DLC overcoat with wear. Underlying the DLCovercoat is a metal (the second material). When the DLC overcoat haswear, the SEM image is brighter at the wear area due a strongerintensity of the underlying metal. In an alternative embodiment, the HDDsubject material is assessed for lubricant pickup. Some embodiments ofthe methods described herein may determine whether the first region isthicker (rather than less thick) than the second region.

As stated in step 204, in an embodiment, background leveling is appliedto the SEM images by top-hat filtering the first region, two-dimensionalspline fitting the first region, and/or flat-frame calibration of thefirst and second region.

Next, as stated in step 206, by image processing the SEM images, adifferential contrast is obtained between the first region and thesecond region. The image processing utilizes the second material todifferentially contrast the first region from the second region.

As stated in step 208, in an embodiment, thickness variation of the SEMimages of the first region and the second region is masked to obtain atwo-dimensional black and white mask including the first region and thesecond region.

Next, as stated in step 210, in an embodiment, contrast variationbetween a series of SEM images of the second region is normalized bymapping an intensity of the series of SEM images of the second regionutilizing linear or low-order fitting, and comparing the series of theSEM images of the second region with an intensity of a reference imageof the second region. Contrast normalization may be used to remove orminimize run-to-run image variation that is not associated with wear. Itmay be used to maintain the integrity of the initial intensity to weardepth calibration. In an example, the average of three or four regionsof the image without wear (e.g., TiC, alumina of the AlTiC and theunworn shield regions) may now be determined.

Next, as stated in step 212, in an embodiment, the thickness variationof the SEM images of the first region and the second region is againmasked to obtain a refined two-dimensional black and white maskincluding the first region and the second region. In an alternativeembodiment, this secondary masking is omitted.

As stated in step 214, in an embodiment, a surface profiler image of thefirst region and the second region is obtained. In an embodiment, atomicforce microscope (AFM) images serve as the surface profiler images ofthe first region and the second region. In an embodiment, alternativesurface profilers may be utilized including but not limited to:variations of AFM such as conductive atomic force microscopy andphotoconductive atomic force microscopy, scanning force microscopy,alternative types of scanning probe microscopy, scanning tunnelingmicroscopy, profilometers, and optical microscopy.

In an embodiment, step 214 and step 216 are performed once or a limitednumber times for the process cycle described in FIG. 2. That is, forexample, surface profiler images (e.g., AFM images) are obtained onceand may serve as a master calibration for numerous SEM images, ratherthan obtaining a surface profiler image for every SEM image, andoverlaying and calibrating a new surface profiler image with every SEMimage.

Next, as stated in step 216, an image intensity variation is determinedbetween the masked SEM images of the first region and the second regionby overlaying and calibrating the SEM images with the surface profilerimages.

As stated in step 218, in an embodiment, the image intensity variationis converted to quantified thickness utilizing a fitted relationobtained from the calibration of the surface profiler images with theSEM images.

In an embodiment, SEM is additionally utilized to quantify area ofthickness variation of the subject material.

Turning now to a representative graph, experimental data is provided toillustrate an example embodiment. Features of the discussion and claimsare not limited to the example embodiment, which is used only forpurposes of the example data. FIG. 3 illustrates an example of SEMimages before background leveling, and after background leveling. Asdescribed in step 204 of FIG. 2, background leveling (e.g., tocompensate for sample tilt) is applied to the SEM images by top-hatfiltering the first region. Alternatively or additionally, backgroundleveling of the SEM images is provided by two-dimensional spline fittingthe wear region, and/or flat-frame calibration of the first region and asecond region. The first region is for example a region having wear, andthe second region is for example a region not having wear. At least someareas of the first region are less thick than the second region.

Flat-frame calibration or flat-field correction is a calibrationprocedure used to improve quality in digital imaging. It removesartifacts from 2-D images caused by variations in the pixel-to-pixelsensitivity of the detector and/or by distortions in the optical path.Flat fielding compensates for different gains and dark currents in adetector. Once a detector has been appropriately flat-fielded, a uniformsignal creates a uniform output. Thus, any further signal is due to thephenomenon being detected and not a systematic error.

In an embodiment, background leveling, wear masking of wear and non-wearregions, and/or contrast normalization is not utilized. Here, sources ofvariation are controlled in an application. For example, the secondregion (e.g., region not having wear) may have consistent imageintensity, and so contrast normalization may be unneeded. In thisembodiment, a method detects thickness variation of a subject material.Images are obtained of a first region of the subject material and asecond region of the subject material utilizing scanning electronmicroscopy (SEM). The SEM images are image processed to obtain adifferential contrast between the first region and the second region.The first region has less thickness than the second region, the subjectmaterial is overlying a second material, and the image processingutilizes the second material to differentially contrast the first regionfrom the second region. Image intensity variation is determined betweenthe masked SEM images of the first region and the second region byobtaining a surface profiler image of the first region and the secondregion, and overlaying and calibrating the SEM images with the surfaceprofiler images. In a further embodiment, image intensity variation isconverted to quantified thickness utilizing a fitted relation obtainedfrom the calibration of the surface profiler images with the SEM images.

FIG. 4 is a representative graph illustrating experimental data of imageintensities mapping for contrast normalization. A newly acquired seriesof the SEM images of the second region are compared with an intensity ofa reference image of the second region. Linear or low-order fitting maybe utilized to map the intensities. As described in step 210 of FIG. 2,contrast variation between a series of SEM images of the second regionis normalized. The first region is for example a region having wear, andthe second region is for example a region not having wear. At least someareas of the first region are less thick than the second region.

Contrast normalization may be utilized for a wide variety of factorsincluding variation of parameters, drift, or the effectiveness of samplegrounding to the SEM. Additionally, in an embodiment, contrastnormalization may be utilized on a surface with uniform wear depth.

Turning now to FIG. 5, an example embodiment output of a pre-processedSEM image, a SEM mask of wear, points of detected wear, and a wear mapis illustrated. The SEM images show a portion of a magnetic head of aHDD with surface wear, and a shield without wear. A diamond-like carbon(DLC) layer is a protective film that is formed on the surfaces of theseregions of the head as a tribological and corrosion barrier for thehead.

The top left image shows a SEM image of the magnetic head that ispreprocessed, as in an embodiment of the invention as described in FIG.2, steps 204 and 206.

The top middle image shows a two-dimensional black and white masked SEMimage of the magnetic head after masking thickness variation between theDLC surface with wear and the DLC surface without wear, as in anembodiment of the invention as described in FIG. 2, step 208. The maskedSEM image reveals regions of the magnetic head that are less thick thanother regions of the magnetic head. In this example, the wear identifiedis wear to the overlying DLC layer. The wear regions appear in white.The shield shows no wear. In this example, the shield is a NiFe alloythat appears as the same intensity. Since Ni and Fe are close in atomicnumber, a change in mixture ratio minimally affects SEM compositionalintensity. A histogram of the image with and without wear may be used tothreshold the image and segment the region with wear from the regionwithout wear.

The top right image is another SEM image showing regions of detectedwear of the magnetic head, and image intensity variation in regionsdelineated by the wear mask.

The bottom left image is a wear map showing quantified wear depth interms of nanometers of the magnetic head. The quantified wear depth isprovided as described in FIG. 2, steps 212-218, after converting theimage intensity variation to a quantified thickness utilizing a fittedrelation obtained from overlaying and the calibrating AFM images of themagnetic head with SEM images of the magnetic head. The wear mapquantifies, in nanometers, wear regions of the magnetic head. In thisexample, the wear identified is wear to the overlying DLC layer. In anembodiment, the fitted relation is a relationship between AFM values andSEM intensity. In an example, the AFM values are in units of nanometers,and the SEM is defined in terms of a matrix of intensity values in 256levels of gray scale corresponding to wear depth. The relationshipbetween AFM values and SEM intensity may be linear, quadratic,polynomial, etc.

FIG. 6 illustrates components of system 300, in an embodiment. System300 includes processor module 304, storage module 306, input/output(I/O) module 308, memory module 310, and bus 302. Although system 300 isillustrated with these modules, other suitable arrangements (e.g.,having more or less modules) known to those of ordinary skill in the artmay be used. For example, system 300 may be a logic implemented statemachine or a programmable logic controller.

In an embodiment, the methods described herein are executed by system300. Specifically, processor module 304 executes one or more sequencesof instructions contained in memory module 310 and/or storage module306. In one example, instructions may be read into memory module 310from another machine-readable medium, such as storage module 306. Inanother example, instructions may be read directly into memory module310 from I/O module 308, for example from an operator via a userinterface. Information may be communicated from processor module 304 tomemory module 310 and/or storage module 306 via bus 302 for storage. Inan example, the information may be communicated from processor module304, memory module 310, and/or storage module 306 to I/O module 308 viabus 302. The information may then be communicated from I/O module 308 toan operator via the user interface.

Memory module 310 may be random access memory or other dynamic storagedevice for storing information and instructions to be executed byprocessor module 304. In an example, memory module 310 and storagemodule 306 are both a machine-readable medium.

In an embodiment, processor module 304 includes one or more processorsin a multi-processing arrangement, where each processor may performdifferent functions or execute different instructions and/or processescontained in memory module 310 and/or storage module 306. For example,one or more processors may execute instructions for image processing SEMimages, and one or more processors may execute instructions forinput/output functions. Also, hard-wired circuitry may be used in placeof or in combination with software instructions to implement variousexample embodiments. Thus, embodiments are not limited to any specificcombination of hardware circuitry and software.

The term “circuit” or “circuitry” as used herein includes all levels ofavailable integration, for example, from discrete logic circuits to thehighest level of circuit integration such as VLSI, and includesprogrammable logic components programmed to perform the functions ofembodiments as well as general-purpose or special-purpose processorsprogrammed with instructions to perform those functions.

Bus 302 may be any suitable communication mechanism for communicatinginformation. Processor module 304, storage module 306, I/O module 308,and memory module 310 are coupled with bus 302 for communicatinginformation between any of the modules of system 300 and/or informationbetween any module of system 300 and a device external to system 300.For example, information communicated between any of the modules ofsystem 300 may include instructions and/or data.

The term “machine-readable medium” as used herein, refers to any mediumthat participates in providing instructions to processor module 304 forexecution. Such a medium may take many forms, including, but not limitedto, non-volatile media, and volatile media. Non-volatile media includes,for example, optical or magnetic disks, such as storage module 306.Volatile media includes dynamic memory, such as memory module 310.Common forms of machine-readable media or computer-readable mediainclude, for example, floppy disk, a flexible disk, hard disk, magnetictape, any other magnetic medium, a CD-ROM, DVD, any other opticalmedium, punch cards, paper tape, any other physical mediums withpatterns of holes, a RAM, a PROM, an EPROM, a FLASH EPROM, any othermemory chip or cartridge, or any other medium from which a processor canread.

In an embodiment, a non-transitory machine-readable medium is employedincluding executable instructions for detecting thickness variation of asubject material.

The instructions include code for obtaining images of a first region anda second region of a subject material, utilizing a scanning electronmicroscopy (SEM), and image processing the SEM images to obtain adifferential contrast between the first region and the second region.The first region has less thickness than the second region. The subjectmaterial is overlying a second material. The image processing utilizesthe second material to differentially contrast the first region from thesecond region. The instructions further include code for determiningimage intensity variation between the masked SEM images of the firstregion and the second region by obtaining a surface profiler image ofthe first region and the second region, and overlaying and calibratingthe SEM images with the surface profiler images. In an embodiment, thesurface profiler images are obtained once or a limited number of timesand may serve as a master calibration for numerous SEM images, ratherthan obtaining a surface profiler image for every SEM image, andoverlaying and calibrating a new surface profiler image with every SEMimage.

In an embodiment, the subject material is at least a portion of amagnetic read head overcoat and/or write head overcoat. In anembodiment, the subject material is a surface of a data storage diskfacing an air bearing surface. In an embodiment, the differentialcontrast is due to differential wear between the first region and thesecond region of the magnetic read head overcoat and/or write headovercoat, wherein the overcoat is diamond-like carbon. In an embodiment,the non-transitory machine-readable medium further includes executableinstructions for background leveling the SEM images by top-hat filteringthe first region, two-dimensional spline fitting the first region,and/or flat-frame calibration the first and second region. In anembodiment, the non-transitory machine-readable medium further includesexecutable instructions for masking thickness variation of the SEMimages of the first region and the second region, to obtain atwo-dimensional black and white mask including the first region and thesecond region. In an embodiment, the non-transitory machine-readablemedium further includes executable instructions for normalizing contrastvariation between a series of SEM images of the second region by mappingan intensity of the series of SEM images of the second region utilizinglinear or low-order fitting, and comparing the series of the SEM imagesof the second region with an intensity of a reference image of thesecond region. In an embodiment, the non-transitory machine-readablemedium further includes executable instructions for secondary maskingthe thickness variation of the SEM images of the first region and thesecond region, to obtain a refined two-dimensional black and white maskincluding the first region and the second region. In an embodiment, thesurface profiler images are atomic force microscope (AFM) images of thefirst region and the second region. In an embodiment, the non-transitorymachine-readable medium further includes executable instructions forconverting image intensity variation to quantified thickness utilizing afitted relation obtained from the calibration of the surface profilerimages with the SEM images. In an embodiment, the non-transitorymachine-readable medium further includes executable instructions forutilizing SEM to quantify area of thickness variation of the subjectmaterial.

Referring to FIG. 7, a representative graph illustrates experimentaldata of quantified wear for example shields S2 and S3, from a Matlabbased algorithm, in an embodiment. Features of the discussion and claimsare not limited to the example embodiment, which is used only forpurposes of the example data. In an embodiment, other programmingbesides Matlab may be utilized.

Sliders with varying degrees of wear from tribological constantoverwrite testing were used. Manual AFM measurements of wear obtainedprior to SEM show acceptable correlation with the degree of wearcalculated from an SEM images process embodiment described above.Correlation with manual AFM measurements also shows acceptablecorrelation.

The images shown are 8-bit SEM images. If error is observed in aparticular regime due to a number of pixels in the SEM images beingsaturated, consequently slightly overestimating the wear amount from SEMimages, 16-bit SEM images may alternatively be utilized to reduce theerror.

The linear equation is shown, and the r-squared value is calculated at0.8562, which is a statistical value describing the accuracy (scale 0.0to 1.0) that one term can be used to predict the value of another term.The r-squared value shows acceptable accuracy for particular embodimentsin assessing quantitative wear depth for a magnetic head for a datastorage device.

Modifications and variations may be made to the disclosed embodimentswhile remaining within the spirit and scope of the method, system andapparatus. The implementations described above and other implementationsare within the scope of the following claims.

We claim:
 1. A method for detecting thickness variation of a subjectmaterial comprising: i.) obtaining images of a first region of thesubject material and a second region of the subject material utilizingscanning electron microscopy (SEM); ii.) image processing the SEM imagesto obtain a differential contrast between the first region and thesecond region, wherein: a.) the first region has less thickness than thesecond region; b.) the subject material is overlying a second material;and c.) the image processing utilizes the second material todifferentially contrast the first region from the second region; andiii.) determining image intensity variation between the masked SEMimages of the first region and the second region by obtaining a surfaceprofiler image of the first region and the second region, and overlayingand calibrating the SEM images with the surface profiler images.
 2. Themethod as in claim 1, wherein the subject material is one of: i.) atleast a portion of a magnetic read head overcoat and/or write headovercoat; and ii.) a surface of a data storage disk facing an airbearing surface.
 3. The method as in claim 2, wherein the differentialcontrast is due to differential wear between the first region and thesecond region of the magnetic read head overcoat and/or write headovercoat, wherein the overcoat is diamond-like carbon.
 4. The method asin claim 1, further comprising background leveling the SEM images by atleast one of: i.) top-hat filtering the first region; ii)two-dimensional spline fitting the first region; and iii.) flat-framecalibration the first and second region.
 5. The method as in claim 1,further comprising masking thickness variation of the SEM images of thefirst region and the second region, to obtain a two-dimensional blackand white mask including the first region and the second region.
 6. Themethod as in claim 5, further comprising normalizing contrast variationbetween a series of SEM images of the second region by mapping anintensity of the series of SEM images of the second region utilizinglinear or low-order fitting, and comparing the series of the SEM imagesof the second region with an intensity of a reference image of thesecond region.
 7. The method as in claim 6, further comprising secondarymasking the thickness variation of the SEM images of the first regionand the second region, to obtain a refined two-dimensional black andwhite mask including the first region and the second region.
 8. Themethod as in claim 1, wherein the surface profiler images are atomicforce microscope (AFM) images of the first region and the second region.9. The method as in claim 1, further comprising converting imageintensity variation to quantified thickness utilizing a fitted relationobtained from the calibration of the surface profiler images with theSEM images.
 10. The method as in claim 1, further comprising utilizingSEM to quantify area of thickness variation of the subject material.